Method for improving quality of spalled material layers

ABSTRACT

Methods for removing a material layer from a base substrate utilizing spalling in which mode III stress, i.e., the stress that is perpendicular to the fracture front created in the base substrate, during spalling is reduced. The substantial reduction of the mode III stress during spalling results in a spalling process in which the spalled material has less surface roughness at one of its&#39; edges as compared to prior art spalling processes in which the mode III stress is present and competes with spalling.

BACKGROUND

The present disclosure relates to semiconductor device manufacturing,and more particularly, to methods for removing a high-quality materiallayer from a base substrate by spalling.

Devices that can be produced in thin-film form have three clearadvantages over their bulk counterparts. First, by virtue of lessmaterial used, thin-film devices ameliorate the materials costassociated with device production. Second, low device weight is adefinite advantage that motivates industrial-level effort for a widerange of thin-film applications. Third, if dimensions are small enough,devices can exhibit mechanical flexibility in their thin-film form.Furthermore, if the substrate from which a device layer is removed canbe reused, additional fabrication cost reduction can be achieved.

Recent advances in spalling techniques now make it possible to remove,i.e., spall, a thin (typically less than 100 μm) material layer from anentire surface of base substrate with near-zero thickness direction kerflosses, and to do this multiple times on the same base substrate. Thepotential cost savings are enormous since (i) the thickness of thespalled material layer can be limited to the thickness needed forthin-film devices, and (ii) many spalled material layers may be derivedfrom a single base substrate.

Further improvements in spalling are however needed which renderspalling more efficient, controllable, and economical and thus morereliable for use in fabricating thin film semiconductor devices.

SUMMARY

The present disclosure provides methods for removing a material layerfrom a base substrate utilizing spalling in which mode III stress, i.e.,a stress component that is perpendicular to the fracture front createdin the base substrate during spalling, is substantially reduced. By“substantially reduced” it is meant that the methods of the presentdisclosure lower the mode III stress within the base substrate duringspalling to a value that is 50% or less as compared to an equivalentprior art method in which mode III stress is present during the spallingprocess. The substantial reduction of the mode III stress duringspalling results in a spalling process in which the spalled material hasless surface roughness at the edges of the spalled material as comparedto prior art spalling processes in which the mode III stress is presentand competes with spalling. A surface roughness reduction of up to afactor of 100 or greater can be achieved near the edge regions of alayer using the methods of the present disclosure.

Applicants have determined through experimentation that mode III stresscan be substantially reduced when spalling is performed under conditionsin which uniaxial stress is the major stress component present duringthe spalling process. When biaxial stress is the major stress componentpresent during the spalling process, mode III stress competes withspalling and the resultant spalled material has an unwanted surfaceroughness near one of its edges.

In one aspect, a method of removing a material layer from a basesubstrate in which mode III stress is substantially reduced is providedthat includes providing a stressor layer with uniaxial stress (i.e.,either intrinsic uniaxial stress or an acquired uniaxial stress using astressor layer having an intrinsic biaxial stress which has beenmodified to be uniaxial) atop a base substrate, wherein the basesubstrate has a fracture toughness that is less than that of thestressor layer. Next, a material layer is removed from the basesubstrate by spalling.

In another aspect, a method of removing a material layer from a basesubstrate in which mode III stress is substantially reduced is providedthat includes providing a structure that includes a stressor layerhaving intrinsic biaxial stress atop a base substrate, wherein the basesubstrate has a fracture toughness that is less than that of thestressor layer. A curvature is then provided to the structure. Next, amaterial layer is removed from the base substrate by spalling, whilemaintaining the curvature in the structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation (through a cross sectional view)illustrating a prior art structure during a conventional spallingprocess in which mode III stress competes with spalling.

FIG. 2 is a pictorial representation (through a top-down) illustrating aprior art structure during a conventional spalling process in which modeIII stress competes with spalling.

FIG. 3 is a pictorial representation (through a cross-sectional view)depicting a base substrate that can be employed in one embodiment of thepresent disclosure.

FIG. 4 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 3 after forming an optionalmetal-containing layer and an optional plating seed layer atop the basesubstrate.

FIG. 5 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 4 after forming a stressor layer havingan intrinsic uniaxial stress atop the base substrate.

FIG. 6 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 5 after forming an optional handlesubstrate atop the stressor layer having an intrinsic uniaxial stress.

FIG. 7 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 6 after spalling a material layer fromthe base substrate.

FIG. 8 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 4 after forming a stressor layer havingan intrinsic biaxial stress atop the structure.

FIG. 9 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 8 after forming a handle substratehaving intrinsic uniaxial stress atop the stressor layer.

FIG. 10 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 9 after spalling a material layer fromthe base substrate.

FIG. 11 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 8 after forming providing an optionalhandle substrate atop the stressor layer having intrinsic biaxial stressand providing a curvature to the resultant structure.

FIG. 12 is a pictorial representation (through a cross sectional view)depicting the structure of FIG. 11 after spalling and releasing thecurvature from the structure.

DETAILED DESCRIPTION

The present disclosure, which provides methods of removing ahigh-quality material layer from a base substrate by spalling, will nowbe described in greater detail by referring to the following discussionand drawings that accompany the present application. It is noted thatthe drawings of the present application are provided for illustrativepurposes and, as such, they are not drawn to scale. In the drawings andthe description that follows, like elements are referred to by likereference numerals. For purposes of the description hereinafter, theterms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”,“top”, “bottom”, and derivatives thereof shall relate to the components,layers and/or elements as oriented in the drawing figures whichaccompany the present application.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide a thoroughunderstanding of the present invention. However, it will be appreciatedby one of ordinary skill in the art that the present disclosure may bepracticed with viable alternative process options without these specificdetails. In other instances, well-known structures or processing stepshave not been described in detail in order to avoid obscuring thevarious embodiments of the present disclosure.

As mentioned above, spalling is one technique that is now available toremove a thin (typically less than 100 μm) material layer from an entiresurface of base substrate. One such prior art spalling process isdisclosed, for example, in U.S. Patent Application Publication No.2010/0311250 to Bedell et al. Specifically, the spalling processdisclosed in the '250 Application includes depositing a stressor layer(i.e., a spall-inducing layer) on a base semiconductor substrate,placing an optional handle substrate on the stressor layer, and inducinga crack and its propagation below the substrate/stressor interface. Atsome stages after spalling, some, or all, of the stressor layer can beremoved utilizing an etch process. In the '250 Application, spallingoccurs in the presence of biaxial stress.

Recently, it was been observed that during the application of acontrolled spalling process using stress inducing layers such asdescribed, for example, in the '250 Application, the surface roughnessof the spalled material located at one edge thereof was higher ascompared with other surface portions of the spalled material. Hence,optimal spalling of the material layer from the base substrate is notachieved utilizing prior art spalling processes in which mode III stressis present and competes with spalling.

One possible explanation of this phenomenon could be that stress actingorthogonal to the fracture propagation direction (arising from thetransition from stressed to non-stressed regions at the wafer edge)creates in combination with the pre-existing mode I and mode IIstresses, and additional mode III stress. The effect of the presence ofnon-zero mode III stress at the advancing fracture front could tend torotate the crack path about an axis parallel with the cleave direction.This competing fracture trajectory may be an origin of the increasedsurface roughness that is observed when typical controlled spallingprocesses are employed.

A fracture front is defined as the boundary between fractured andnon-fractured material (i.e., the crack) and has a direction defined bythe instantaneous direction of fracture. A material undergoing fracturecan be subjected to three principle stresses; mode I (or opening mode),mode II (shear mode) and mode III. In mode I, stress is applied in adirection normal to the plane of fracture. In mode II, stress is appliedin shear parallel to the direction of fracture. In mode III, stress isapplied in shear perpendicular to the direction of fracture. Spallingmode fracture is a phenomenon that occurs when both mode I and mode IIare present simultaneously. The mode II stress deflects the advancingcrack tip either downward into the substrate (for a tensile stressorlayer) or upwards towards the surface (for a compressive stressor).

The effect of mode III stress is to rotate the crack front about an axisparallel with the fracture direction. When the stressor is under biaxialtensile stress, mode III stress is present at the edges of the substratethat have boundary normal vectors orthogonal to the fracture direction(position C in FIG. 2) and therefore has higher surface roughness. Whenthe boundary normal vector is parallel to the fracture direction(position B in FIG. 2) only mode I and mode II stresses are present andthe surface is smoother. Using a biaxial stressor, mode III stress isalso present along the fracture front when curvature is present and isshown schematically in position C in FIG. 2.

Referring to FIG. 1, there is shown a cross-sectional view of a priorart structure including a base substrate 100 and a stressor layer 102.In this example, spalling is performed in the presence of mode IIIstress due to the biaxial stress that is present in the originalstressor layer 102. Since mode III stress is present during spalling thestress direction is perpendicular, i.e., orthogonal, to the crackdirection. As such, the crack that is formed will rotate out of theplane and will compete with spalling at the edge of the base substrate.This results in unwanted surface roughness at the edges of the spalledmaterials. In FIG. 1, the stress is biaxial and therefore into (and outof) the page as indicated by the arrow-tail symbol as well as directedin from the edge (for tensile stress). Also in FIG. 1 the crackpropagation direction is into the page as indicated by the arrow-tailsymbol. FIG. 2 illustrates a top view of a same type prior art structureduring spalling showing the spalled region, and the non-spalled region.The arrow designated as A is the fracture front and has a directiondefined by the fracture propagation direction, the arrow designed as Bis the location of fracture initiation and has a boundary normal vectorparallel to the fracture propagation direction and the arrow designed asC is the boundary that is perpendicular to the fracture propagation. Itis noted that the mode III stress described above is identified by arrowC as well.

Because the existence of a stress discontinuity at the stress edge is acertainty, a method of reducing transverse stress, i.e., mode IIIstress, along the film edges orthogonal to the fracture direction isrequired. Additionally, instability in a propagating fracture front,such as curvature, will tend to create additional roughness by the samemechanism.

In view of the above, and as stated previously, the present disclosuresprovides various methods of spalling a material layer from a basesubstrate wherein the spalling process is performed under conditions inwhich mode III stress, i.e., the stress that is perpendicular to thefracture front created in the base substrate during spalling, isreduced. The substantial reduction of the mode III stress duringspalling results in a spalling process in which the spalled material hasless surface roughness at its' edges as compared to prior art spallingprocesses in which the mode III stress is present and competes withspalling. As such, the present disclosure provides a high quality, interms of a reduced surface roughness, spalled material layer.

Applicants have determined through experimentation that mode III stresscan be substantially reduced when spalling is performed under conditionsin which uniaxial stress is the major stress component present duringthe spalling process. When biaxial stress is the major stress componentpresent during the spalling process, mode III stress competes withspalling and the resultant spalled material has an unwanted surfaceroughness near one of its' edges. It is also contemplated herein thatthe direction of uniaxial stress corresponds to preferred fracturepropagation directions as described in U.S. Patent ApplicationPublication No. 2010/0311250 to Bedell et al. For example, the primarydirection of the uniaxial stress can be made to be aligned to any of the4 [100] directions when a <001> crystal serves as the base substrate 10.

Referring first to FIG. 3, there is shown a base substrate 10 having anuppermost surface 12 that can be employed in one embodiment of thepresent disclosure. The base substrate 10 that can be employed in thepresent disclosure may comprise a semiconductor material, a glass, aceramic, or any another material whose fracture toughness is less thanthat of the stressor layer to be subsequently formed.

Fracture toughness is a property which describes the ability of amaterial containing a crack to resist fracture. Fracture toughness isdenoted K_(Ic). The subscript Ic denotes mode I crack opening under anormal tensile stress perpendicular to the crack, and c signifies thatit is a critical value. Mode I fracture toughness is typically the mostimportant value because spalling mode fracture usually occurs at alocation in the substrate where mode II stress (shearing) is zero.Fracture toughness is a quantitative way of expressing a material'sresistance to brittle fracture when a crack is present.

When the base substrate 10 comprises a semiconductor material, thesemiconductor material may include, but is not limited to, Si, Ge, SiGe,SiGeC, SiC, Ge alloys, GaSb, GaP, GaN, GaAs, InAs, InP, and all otherIII-V or II-VI compound semiconductors. In some embodiments, the basesubstrate 10 is a bulk semiconductor material. In other embodiments, thebase substrate 10 may comprise a layered semiconductor material such as,for example, a semiconductor-on-insulator or a semiconductor on apolymeric substrate. Illustrated examples of semiconductor-on-insulatorsubstrates that can be employed as base substrate 10 includesilicon-on-insulators and silicon-germanium-on-insulators.

When the base substrate 10 comprises a semiconductor material, thesemiconductor material can be doped, undoped or contain doped regionsand undoped regions.

In one embodiment, the semiconductor material that can be employed asthe base substrate 10 can be single crystalline (i.e., a material inwhich the crystal lattice of the entire sample is continuous andunbroken to the edges of the sample, with no grain boundaries). Inanother embodiment, the semiconductor material that can be employed asthe base substrate 10 can be polycrystalline (i.e., a material that iscomposed of many crystallites of varying size and orientation; thevariation in direction can be random (called random texture) ordirected, possibly due to growth and processing conditions). In yetanother embodiment of the present disclosure, the semiconductor materialthat can be employed as the base substrate 10 can be amorphous (i.e., anon-crystalline material that lacks the long-range order characteristicof a crystal). Typically, the semiconductor material that can beemployed as the base substrate 10 is a single crystalline material.

When the base substrate 10 comprises a glass, the glass can be aSiO₂-based glass which may be undoped or doped with an appropriatedopant. Examples of SiO₂-based glasses that can be employed as the basesubstrate 10 include undoped silicate glass, borosilicate glass,phosphosilicate glass, fluorosilicate glass, and borophosphosilicateglass.

When the base substrate 10 comprises a ceramic, the ceramic can be anyinorganic, non-metallic solid such as, for example, an oxide including,but not limited to, alumina, beryllia, ceria and zirconia, a non-oxideincluding, but not limited to, a carbide, a boride, a nitride or asilicide; or composites that include combinations of oxides andnon-oxides.

In some embodiments of the present disclosure, one or more devicesincluding, but not limited to, transistors, capacitors, diodes, BiCMOS,resistors, etc. can be processed on and/or within the uppermost surface12 of the base substrate 10 utilizing techniques well known to thoseskilled in the art.

In some embodiments of the present disclosure, the uppermost surface 12of the base substrate 10 can be cleaned prior to further processing toremove surface oxides and/or other contaminants therefrom. In oneembodiment of the present disclosure, the base substrate 10 is cleanedby applying to the base substrate 10 a solvent such as, for example,acetone and isopropanol, which is capable of removing contaminatesand/or surface oxides from the uppermost surface 12 of the basesubstrate 10.

Referring to FIG. 4, there is depicted the base substrate 10 of FIG. 3after forming an optional metal-containing adhesion layer 14 and anoptional plating seed layer 15 atop the base substrate 10. In someembodiments, at least one of the optional metal-containing adhesionlayer 14 and the optional plating seed layer 15 is employed. In otherembodiments, both the optional metal-containing adhesion layer 14 andthe optional plating seed layer 15 are employed. In another embodiment,neither the optional metal-containing adhesion layer 14, nor theoptional plating seed layer 15 is employed.

The optional metal-containing adhesion layer 14 is employed inembodiments in which the stressor layer to be subsequently formed haspoor adhesion to uppermost surface 12 of base substrate 10. Typically,the metal-containing adhesion layer 14 is employed when ametal-containing stressor layer is employed.

The optional metal-containing adhesion layer 14 which can be employed inthe present disclosure includes any metal adhesion material such as, butnot limited to, Ti/W, Ti, Cr, Ni or any combination thereof. Theoptional metal-containing adhesion layer 14 may comprise a single layeror it may include a multilayered structure comprising at least twolayers of different metal adhesion materials.

The metal-containing adhesion layer 14 that can be optionally formed onthe uppermost surface 12 of base substrate 10 can be formed at roomtemperature (15° C.-40° C.) or above. In one embodiment, the optionalmetal-containing adhesion layer 14 can be formed at a temperature whichis from 20° C. to 180° C. In another embodiment, the optionalmetal-containing adhesion layer 14 can be formed at a temperature whichis from 20° C. to 60° C.

The metal-containing adhesion layer 14, which may be optionallyemployed, can be formed utilizing deposition techniques that are wellknown to those skilled in the art. For example, the optionalmetal-containing adhesion layer 14 can be formed by sputtering, chemicalvapor deposition, plasma enhanced chemical vapor deposition, chemicalsolution deposition, physical vapor deposition, and plating. Whensputter deposition is employed, the sputter deposition process mayfurther include an in-situ sputter clean process before the deposition.

When employed, the optional metal-containing adhesion layer 14 typicallyhas a thickness of from 5 nm to 200 nm, with a thickness of from 50 nmto 150 nm being more typical. Other thicknesses for the optionalmetal-containing adhesion layer 14 that are below and/or above theaforementioned thickness ranges can also be employed in the presentdisclosure.

The optional plating seed layer 15 is employed in embodiments in whichthe stressor layer to be subsequently formed is a metal and plating isused to form the metal-containing stressor layer. The optional platingseed layer 15 is employed to selectively promote subsequent plating of apre-selected metal-containing stressor layer. The optional plating seedlayer 15 may comprise, for example, single layer of Ni or a layeredstructure of two or more metals such as Al(bottom)/Ti/Ni(top).

The thickness of the optional seed layer 15 may vary depending on thematerial or materials of the optional plating seed layer 15 as well asthe technique used in forming the same. Typically, the optional platingseed layer 15 has a thickness from 2 nm to 400 nm. The optional platingseed layer 15 can be formed by a conventional deposition processincluding, for example, chemical vapor deposition (CVD), plasma-enhancedchemical vapor deposition (PECVD), atomic layer deposition (ALD), andphysical vapor deposition (PVD) techniques that may include evaporationand/or sputtering.

Referring now to FIG. 5, there is illustrated the structure of FIG. 4after forming a stressor layer 16 having an intrinsic uniaxial stressatop the base substrate 10. In one embodiment (not shown), the stressorlayer 16 can be formed directly on an upper surface of the optionalmetal-containing adhesion layer 14. In some embodiments and asillustrated in FIG. 5, the stressor layer 16 can be formed directly onan upper surface of the optional plating seed layer 15. In someembodiments in which the optional metal-containing adhesion layer 14 andthe optional plating seed layer 15 are not present, the stressor layer16 can be formed directly on the uppermost surface 12 of base substrate10; this particular embodiment is not shown in the drawings, but canreadily be deduced from the drawings illustrated in the presentapplication.

The term “intrinsic uniaxial stress” is used throughout the presentapplication to denote that the material has an inherent state of stressin which two of the three principal stress vectors are zero. As such,the stress within the material is in one direction. The stressor layer16 having uniaxial stress that can be employed in this embodiment of thepresent disclosure includes any material that is under tensile stress onbase substrate 10 at the spalling temperature. As such, the stressorlayer 16 can also be referred to herein as a stress-inducing layer.

In accordance with the present disclosure, the stressor layer 16 has acritical thickness and stress value that cause spalling mode fracture tooccur within the base substrate 10. By “spalling mode fracture” it ismeant that a crack is formed within base substrate 10 and thecombination of loading forces maintains a crack trajectory at a depthbelow the stressor/substrate interface. By critical condition, it ismeant that for a given stressor material and base substrate materialcombination, a thickness value and a stressor value for the stressorlayer is chosen that render spalling mode fracture possible (can producea K_(I) value greater than the K_(IC) of the substrate). Moreover, modeIII stress is substantially reduced since spalling will be performed inthe presence of a uniaxial stress load.

The thickness of the stressor layer 16 having intrinsic uniaxial stressis chosen to provide the desired fracture depth within the basesubstrate 10. For example, if the stressor layer 16 having intrinsicuniaxial stress is chosen to be Ni, then fracture will occur at a depthbelow the stressor layer 16 roughly 2 to 3 times the Ni thickness. Thestress value for the stressor layer 16 having intrinsic uniaxial stressis then chosen to satisfy the critical condition for spalling modefracture. This can be estimated by inverting the empirical equationgiven by t*=[(2.5×10⁶)(K_(IC) ^(3/2))]/σ², where t* is the criticalstressor layer thickness (in microns), K_(IC) is the fracture toughness(in units of MPa·m^(1/2)) of the base substrate 10 and a is the stressvalue of the stressor layer (in MPa or megapascals). The aboveexpression is a guide, in practice, spalling can occur at stress orthickness values up to 20% less than that predicted by the aboveexpression.

Illustrative examples of materials that are under tensile stress whenapplied atop the base substrate 10 and thus can be used as the stressorlayer 16 having intrinsic uniaxial stress include, but are not limitedto, a metal, a polymer, such as a spall inducing tape layer, or anycombination thereof. The stressor layer 16 having intrinsic uniaxialstress may comprise a single stressor layer, or a multilayered stressorstructure including at least two layers of different stressor materialcan be employed.

In one embodiment, the stressor layer 16 having intrinsic uniaxialstress is a metal. In another embodiment, the stressor layer 16 havingintrinsic uniaxial stress is a spall inducing tape. In anotherembodiment, for example, the stressor layer 16 having intrinsic uniaxialstress may comprise a two-part stressor layer including a lower part andan upper part. The upper part of the two-part stressor layer can becomprised of a spall inducing tape layer.

When a metal is employed as the stressor layer 16 having intrinsicuniaxial stress, the metal can include, for example, Ni, Cr, Fe, Mo, orW. Alloys of these metals can also be employed. In one embodiment, thestressor layer 16 having intrinsic uniaxial stress includes at least onelayer consisting of Ni.

When a polymer is employed as the stressor layer 16 having intrinsicuniaxial stress, the polymer is a large macromolecule composed ofrepeating structural units. These subunits are typically connected bycovalent chemical bonds. Illustrative examples of polymers that can beemployed as the stressor layer 16 having intrinsic uniaxial stressinclude, but are not limited to, polyimides polyesters, polyolefins,polyacrylates, polyurethane, polyvinyl acetate, and polyvinyl chloride.

When a spall inducing tape layer is employed as the stressor layer 16having intrinsic uniaxial stress, the spall inducing tape layer includesany pressure sensitive tape that is flexible, soft, and stress free at afirst temperature used to form the tape, yet strong, ductile and tensileat a second temperature used during removal of the upper portion of thebase substrate 10. By “pressure sensitive tape,” it is meant an adhesivetape that will stick with application of pressure, without the need forsolvent, heat, or water for activation. Tensile stress in the tape atthe second temperature is primarily due to thermal expansion mismatchbetween the base substrate 10 (with a lower thermal coefficient ofexpansion) and the tape (with a higher thermal expansion coefficient).

Typically, the pressure sensitive tape that is employed in the presentdisclosure as stressor layer 16 includes at least an adhesive layer anda base layer. Materials for the adhesive layer and the base layer of thepressure sensitive tape include polymeric materials such as, forexample, acrylics, polyesters, olefins, and vinyls, with or withoutsuitable plasticizers. Plasticizers are additives that can increase theplasticity of the polymeric material to which they are added.

In one embodiment, the stressor layer 16 having intrinsic uniaxialstress employed in the present disclosure can be formed at a firsttemperature which is at room temperature (15° C.-40° C.). In anotherembodiment, when a tape layer is employed, the tape layer can be formedat a first temperature which is from 15° C. to 60° C.

In one embodiment, and when the stressor layer 16 is a metal, the metalfilm having intrinsic uniaxial stress can be formed by sputtering ametal atom from a metal target while rotating the base substrate 10and/or the target. In another embodiment, it may be possible tomanipulate the shape and size of metal film grains, grain boundariesand/or voids (as well as any other structural features affecting filmstress) with the use of patterned plating seed layers during metalstressor film growth. Electroplating above the exposed plating seedlayer would have a growth direction perpendicular to the upper surfaceof the base substrate, while lateral overgrowth plating over aninsulating line would have a growth direction parallel to the seedsurface. In another embodiment, a metal stressor film having intrinsicuniaxial stress can be formed by sputtering a metal that has one grainstructure over the lines and another between the lines, or grainboundaries aligned with line edges. The lines and spaces may be formedby conventional lithographic techniques as well as direct writetechniques or self-assembly block copolymer technology.

In another embodiment, a metal stressor film having intrinsic uniaxialstress can be formed by sputtering a metal while the substrate 10 issubjected to cylindrical curvature by mechanical means, such as, forexample, a mechanical jig or vice. In this embodiment, the initiallybiaxial stressed stressor layer 16 is converted into uniaxial stress asthe substrate 10 is released from the mechanical jig or vice. The radiusof curvature of the base substrate 10 during deposition of stressorlayer 16 can be from 0.2 meters to 10 meters. The lower radius ofcurvature corresponds to thinner base substrate 10 thickness values.

When the stressor layer 16 having intrinsic uniaxial stress is apolymer, an initial polymeric stressor layer can be formed by adeposition technique including, for example, dip coating, spin-coating,brush coating, sputtering, chemical vapor deposition, plasma enhancedchemical vapor deposition, chemical solution deposition, and physicalvapor deposition. In some instances, the deposited polymeric materialmay have the desired intrinsic uniaxial stress. In other embodiments,intrinsic uniaxial stress can be provided to the polymeric film bystretching the film prior to application.

When the stressor layer 16 is a spall inducing tape layer, the tapelayer can be applied by hand or by mechanical means to the structure.The spall inducing tape can be formed utilizing techniques well known inthe art or they can be commercially purchased from any well knownadhesive tape manufacturer. Some examples of spall inducing tapes thatcan be used in the present disclosure as stressor layer 16 include, forexample, Nitto Denko 3193MS thermal release tape, Kapton KPT-1, andDiversified Biotech's CLEAR-170 (acrylic adhesive, vinyl base). In someembodiments, the spalling inducing tape has intrinsic uniaxial stress.In other embodiments, intrinsic uniaxial stress can be provided to thetape by stretching the tape prior to application.

In one embodiment, a two-part stressor layer can be formed on a surfaceof the base substrate 10, wherein a lower part of the two-part stressorlayer is formed at a first temperature which is at room temperature orslight above (e.g., from 15° C. to 60° C.), wherein an upper part of thetwo-part stressor layer comprises a spall inducing tape layer at anauxiliary temperature which is at room temperature.

If the stressor layer 16 is of a metallic nature, it typically has athickness of from 3 μm to 50 μm, with a thickness of from 4 μm to 7 μmbeing more typical. Other thicknesses for the stressor layer 16 that arebelow and/or above the aforementioned thickness ranges can also beemployed in the present disclosure.

If the stressor layer 16 is of a polymeric nature, it typically has athickness of from 10 μm to 200 μm, with a thickness of from 50 μm to 100μm being more typical. Other thicknesses for the stressor layer 16 thatare below and/or above the aforementioned thickness ranges can also beemployed in the present disclosure.

Referring to FIG. 6, there is depicted the structure of FIG. 5 afterforming an optional handle substrate 18 atop stressor layer 16 havinguniaxial stress. The optional handle substrate 18 employed in thisembodiment of the present disclosure comprises any flexible materialwhich is either non-stressed or has some stress, e.g., uniaxial stress,which does not affect the nature of the stressor layer 16 havingintrinsic uniaxial stress. Illustrative examples of flexible materialsthat can be employed as the optional handle substrate 18 include a metalfoil or a polyimide foil. The optional handle substrate 18 can be usedto provide better fracture control and more versatility in handling thespalled material. Moreover, the optional handle substrate 18 can be usedto guide the crack propagation during spalling. The optional handlesubstrate 18 of the present disclosure is typically, but notnecessarily, formed at a first temperature which is at room temperature(15° C.-40° C.).

The optional handle substrate 18 can be formed utilizing depositiontechniques that are well known to those skilled in the art including,for example, dip coating, spin-coating, brush coating, sputtering,chemical vapor deposition, plasma enhanced chemical vapor deposition,chemical solution deposition, physical vapor deposition, and plating.The optional handle substrate 18 typical has a thickness of from 1 μm tofew mm, with a thickness of from 70 μm to 120 μm being more typical.Other thicknesses for the optional handle substrate 18 that are belowand/or above the aforementioned thickness ranges can also be employed inthe present disclosure.

Referring now to FIG. 7, there is depicted the structure of FIG. 6 afterremoving a material layer 10′ from the base substrate 10 by spalling. Inthis and other embodiments of the present disclosure, spalling isperformed in the present of uniaxial stress, not biaxial stress as isthe case in prior art spalling processes. As a result, mode III stressis substantially reduced and the resultant spalled materials have lesssurface roughness at the edges of the spalled material. Spalling can beinitiated at room temperature or at a temperature that is less than roomtemperature. In one embodiment, spalling is performed at roomtemperature (i.e., 20° C. to 40° C.). In another embodiment, spalling isperformed at a temperature less than 20° C. In a further embodiment,spalling occurs at a temperature of 77 K or less. In an even furtherembodiment, spalling occurs at a temperature of less than 206 K. Instill yet another embodiment, spalling occurs at a temperature from 175K to 130 K.

When a temperature that is less than room temperature is used, the lessthan room temperature spalling process can be achieved by cooling thestructure down below room temperature utilizing any cooling means. Forexample, cooling can be achieved by placing the structure in a liquidnitrogen bath, a liquid helium bath, an ice bath, a dry ice bath, asupercritical fluid bath, or any cryogenic environment liquid or gas.

When spalling is performed at a temperature that is below roomtemperature, the spalled structure is returned to room temperature byallowing the spalled structure to slowly cool up to room temperature byallowing the same to stand at room temperature. Alternatively, thespalled structure can be heated up to room temperature utilizing anyheating means.

After spalling, the optional handle substrate 18, stressor layer 16,and, if present the optional plating seed layer 15 and the optionalmetal-containing adhesion layer 14 can be removed from the spalledmaterial layer 10′. The remaining portion of base substrate afterspalling is labeled as 11. The optional handle substrate 18, thestressor layer 16, the optional plating seed layer 15 and the optionalmetal-containing adhesion layer portion 14 can be removed from thematerial layer 10′ that was spalled from base substrate 10 utilizingconventional techniques well known to those skilled in the art. Forexample, and in one embodiment, aqua regia (HNO₃/HCl) can be used forremoving the optional handle substrate 18, the stressor layer 16, theoptional plating seed layer 15, and the optional metal-containingadhesion layer 14. In another example, UV or heat treatment is used toremove the optional handle substrate 18, followed by a chemical etch toremove the stressor layer 16, followed by a different chemical etch toremove the optional plating seed layer 15, and optional metal-containingadhesion layer 14.

The thickness of the material layer 10′ that is spalled from the basesubstrate 10 varies depending on the material of the stressor layer 16and the material of the base substrate 10 itself. In one embodiment, thematerial layer 10′ that is spalled from the base substrate 10 has athickness of less than 100 microns. In another embodiment, the materiallayer 10′ that is spalled from the base substrate 10 has a thickness ofless than 50 microns.

Referring now to FIG. 8, there is depicted the structure of FIG. 4 afterforming a stressor layer 16′ having an intrinsic biaxial stress atop thestructure. The stressor layer 16′ having the intrinsic biaxial stresscan be formed directly atop one of the optional seed layer 15, theoptional metal-containing adhesion layer 14, or the base substrate 10.

The stressor layer 16′ having the intrinsic biaxial stress can includeone of the materials (i.e., metal and/or polymers) mentioned above forthe stressor layer 16 having the intrinsic uniaxial stress. The term“intrinsic biaxial stress” denotes that the material has an inherentstate of stress including three mutually perpendicular principal stressvectors, wherein two of the stress vectors act in the same plane and oneis zero. The stressor layer 16′ having the intrinsic biaxial stress canhaving a thickness within one of the ranges mentioned above for thestressor layer 16 having the intrinsic uniaxial stress. When polymersand tapes are used as the stressor layer 16′ having the intrinsicbiaxial stress, the polymers and type can be formed as described abovefor the stressor layer 16 having the intrinsic uniaxial stress. When ametal is employed as the stressor layer 16′ having the intrinsic biaxialstress, the metal can be formed by a deposition technique including, forexample, dip coating, spin-coating, brush coating, sputtering, chemicalvapor deposition, plasma enhanced chemical vapor deposition, chemicalsolution deposition, and physical vapor deposition.

Referring now to FIG. 9, there is depicted the structure of FIG. 8 afterforming a handle substrate 18′ having intrinsic uniaxial stress atop thestressor layer 16′. In this embodiment, the handle substrate 18′includes one of the materials mentioned above for handle substrate 18.The handle substrate 18′ can be formed utilizing one of techniquesmentioned above for handle substrate 18. It is noted that in thisembodiment (as well as any of the other embodiments of the presentdisclosure), the optional plating seed layer 15 and/or the optionalmetal-containing adhesion layer 14 can be omitted.

Referring to FIG. 10, there is depicted the structure of FIG. 9 afterspalling a material layer 10′ from the base substrate 10. Spalling canbe performed as described above. In this embodiment, the net effect ofutilizing a stressor layer that has an intrinsic biaxial stress and ahandle substrate having intrinsic uniaxial stress is that spalling isessentially carried out with a stressor layer with an uniaxial stress.In this embodiment, the handle substrate 18′, the stressor layer 16′,the optional plating seed layer 15, and the optional metal-containingadhesion layer 4 can be removed from the spalled material layer 10′ asdescribed above.

Referring now to FIG. 11, there is depicted the structure of FIG. 8after providing an optional handle substrate 18 atop the stressor layer16′ and providing a curvature to the resultant structure. In thisembodiment of the present disclosure, the curvature can be provided tostructure by hand or by mechanical means, such as, for example, amechanical jig or vice. The radius of curvature that is provided thestructure including at least the base substrate 10 and stressor layer16′ can be from 0.2 to 10 meters (smaller radius of curvature for thinsubstrates).

Referring now to FIG. 12, there is illustrated the structure of FIG. 11after spalling and releasing the curvature from the structure. Spallingis achieved as described above.

It is noted that in the various embodiments depicted in FIGS. 8-12 meansare provided which essentially modify (alter) stressor layer 16′ to astressor layer with uniaxial stress. This is achieved by basicallycancelling out one of the direction of one of the stress vectors.

While the present disclosure has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed is:
 1. A method of removing a material layer from a basesubstrate, said method comprising: providing a stressor layer withuniaxial stress atop a base substrate having a fracture toughness thatis less than that of said stressor layer, wherein said providing saidstressor layer with uniaxial stress comprises forming a material havingintrinsic uniaxial stress atop said base substrate; and spalling amaterial layer from said base substrate.
 2. The method of claim 1,wherein said forming the material having said intrinsic uniaxial stresscomprises sputtering a metal atom from a metal target while rotatingsaid base substrate.
 3. The method of claim 1, wherein said forming saidmaterial having said intrinsic uniaxial stress comprises providing apatterned plating seed layer on said base substrate and thenelectrodepositing a metal as said stressor layer.
 4. The method of claim1, wherein said forming said material having said intrinsic uniaxialstress comprises providing insulating lines on said base substrate andsputtering a metal that has one grain structure over said insulatinglines and between each insulating line.
 5. The method of claim 1,further comprising providing a handle substrate atop the stressor layerprior to spalling.
 6. The method of claim 1, wherein said spalling isperformed at room temperature or a temperature of less than roomtemperature.
 7. The method of claim 1, wherein said base substratecomprises a semiconductor material.
 8. The method of claim 1, furthercomprising forming a metal-containing adhesion layer between said basesubstrate and said stressor layer.
 9. The method of claim 1, furthercomprising forming a plating seed layer on said metal-containingadhesion layer.
 10. The method of claim 1, wherein said stressor layercomprises a metal selected from the group consisting of Ni, Cr, Fe, Moand W.
 11. The method of claim 1, further comprising removing saidstressor layer from said material layer after spalling.
 12. The methodof claim 11, wherein said removing said stressor layer comprises achemical etch.
 13. The method of claim 12, wherein said chemical etchcomprises a mixture of HNO_(s) and HCl.